Reverse units are designed to pump fluid into a disposal well. Typically this well is used to permanently dispose of fluids, or to help push desired products towards a producing well.

The fluids used in permanent disposal often contain concentrations of fluids or material that may be damaging on the earth's surface. Concentrations of salts, minerals, radioactive material, acids, or bases may be environmentally damaging to animals and plants. Pushing them into underground caverns prevents them from harming wildlife and humans. Since underground caverns often contain similar fluids, filling underground caverns is not considered to be environmentally damaging.

For well stimulation applications, fluids are pumped into wells nearby or surrounding a producing well. The fluids then travel through the ground and help to loosen and push desired products to the production well. Depending on the rock layers, desired fluids, and other factors, fluids may be heated or of different types of compositions.

Reverse units are comprised of several pieces of equipment. This equipment typically includes a driver, transmission and/or gear box, and a pump. Most reverse units use a diesel engine, clutch, transmission and a positive displacement pump. In some cases, reverse units using electric motors or centrifugal pumps are used.

The use of positive displacement pumps and transmissions allow for the control of volume being pumped into the well. Positive displacement pumps are able to provide a continuous, set flow rate as long as the pressure remains below safe levels. Since pressures in a disposal well can fluctuate, a fixed flow rate is often viewed as beneficial for these applications.

Control systems for reverse units have a variety of options. Less expensive units are fully manual, requiring an operator to be present at all times. More expensive units can be integrated into automatic and wireless systems, allowing for the operation of multiple units from a central location, with or without operators. Most reverse units on the market use a balance between the two options. Safety shut-downs such as vibration switches, low oil switches, and temperature switches are used to protect the unit and operator and usually come standard on the pump units. It is not necessarily required for these systems to be automatic shutdowns, but it is usually beneficial for added protection of the equipment. These shutdowns will detect and react before an operator is able to notice a problem is occurring.

In all cases, a relief valve is required for the safety of equipment and personnel. If the well pressure were to spike, a relief valve provides a direction of flow to relieve harmful pressure. In positive displacement pumps, the pump is able to provide pressure beyond its material strength. These pumps have the capability of continuing to produce pressures through catastrophic failure. Placing a relief valve back to suction or atmospheric pressure will prevent accidental pressure spikes that may damage equipment and harm operators.